This invention relates polymeric materials used in air-cooled generators and a method of forming same, and more particularly, this invention relates to polymeric materials used with electrical insulation, tapes, coatings and similar applications which are more resistant than conventional materials to the effects of corrosive byproducts, e.g., ozone, peroxy free radicals, produced by partial discharges within an electrical generator.
One critical factor that determines the long-term performance of electrical insulation and other materials used in air-cooled generators is the stability of the polymeric materials to thermal, mechanical and electrical stresses. The resistance of organic polymeric materials to oxidative degradation, particularly at elevated temperatures, is a key requirement for obtaining satisfactory, long-term performance.
Some polymers, however, oxidize more readily than others. For example, high aliphatic content polyesters tend to degrade more readily than epoxies, resulting in a loss of key properties, such as adhesion, flexibility, tensile strength, dielectric strength, and other similar properties. Thermal oxidative degradation of polymeric insulation materials also leads to degraded operating properties in motor and generator insulation, including partial discharges, increasing tan ∂ xe2x80x9ctip-upxe2x80x9d of delamination, fracture and embrittlement. The attack from ozone and other reactive peroxy radical species resulting from partial discharges in the generator are particularly harmful.
Some electrical utilities have aggressively tested these electrical components under thermal stressed conditions. These studies have shown that under aggressive conditions, in an air ambient, premature coil degradation can occur, as evidenced by decreased voltage-loss tangent tip-up and increased power factor measurements. If failure is defined as a measured decrease in tip-up and increased power factor, then these coils have experienced an increased failure rate under these more stressful conditions.
There is also a concern with electric utilities and power generator manufacturers that the epoxy resins tend to degrade in air via an xe2x80x9cautocatalyticxe2x80x9d free radical chain process where runaway degradation occurs. There is also a potential for the high electric fields to enhance this degradation process via discharge and polarization effects. This enhances electrochemically induced reactions. The present uncertainty of the chemical reactions in air is highlighted by a possible competing mechanism where polarization effects may retard degradation. For example, some prior art studies have noted the aging of epoxy based rotor insulation in air (20% O2) and in 5% O2.
The exact mechanism that is responsible for the thermo-oxidative degradation of resins, such as epoxies, is not understood in great detail. It is generally believed, however, by many skilled in the art, that the chemical species responsible for thermal degradation are free radicals, i.e., very reactive transient compounds, which are derived from atmospheric oxygen and breakdown fragments from the polymer resin. In the case of a bisphenol xe2x80x9cAxe2x80x9d epoxy resin, it has been found that the 
ether linkage and isopropylidene group are weak links 
for free radical attack. It has been found that the combination of temperature and oxygen produces an xe2x80x9cautocatalyticxe2x80x9d effect, which causes more rapid polymer degradation than some thermal effects. By controlling or xe2x80x9ctrappingxe2x80x9d the free radical species responsible for degradation, improved thermo-oxidative stability composition of polymers could be developed, such as epoxies and polyesters.
It has been known to add xe2x80x9cantioxidantxe2x80x9d additives for controlling oxidation of polymers and plastics. These additives have been used commercially in different materials, such as polyethylene, polypropylene, neoprene, and other thermoplastics and elastomers. They have not had widespread use in thermoset materials, e.g., polyesters and epoxies, because of a perception that thermosets are more resistant to oxidative attack. Although this may be true for xe2x80x9cnormalxe2x80x9d use, it is not true for the harsher, stressful environments found with high voltage electrical insulation, particularly with ozone present in elevated temperature oxidative environments of air-cooled generators.
Any selected antioxidant materials should be xe2x80x9ccompatiblexe2x80x9d with insulation resins such as epoxies and polyesters. xe2x80x9cCompatiblexe2x80x9d would indicate that these additives have no deleterious effect on the long-term performance and properties of insulation polymers (e.g., electrical, mechanical, and chemical properties). These additives should also not have any adverse effect on the processing characteristics of the resins, for example, the gel time, viscosity, tank stability, post-cure and xe2x80x9cwettingxe2x80x9d of mica. Most commercially available antioxidants would have adverse effects on these polymers. This would be particularly true of amine and organometallic based compounds.
Another problem associated with common, prior art antioxidants is migration from the polymer composition over time because of elevated temperatures, causing the polymer to lose its protection. This migration of antioxidant, however, can be controlled by chemically bonding the antioxidant to the polymer structure and preventing loss of the additive.
For suitability in high voltage insulation, any antioxidants should have the following characteristics:
1. They should be compatible with the resin and have no deleterious effect on long-term performance or on the processing characteristics.
2. They should be xe2x80x9cnon-fugitivexe2x80x9d in nature, i.e., they should not migrate out of the polymer structure.
3. They should be easily reacted into the polymer structure before final processing and the cure of the insulation resin.
4. They should be used at the lowest possible concentrations preferably less than 0.5% w/w, to minimize the effects on electrical properties, including the dielectric constant and dissipation factor, and tensile properties, including tensile strength and flexural modulus.
The present invention is advantageous and an improved insulated electrical coil formed as a plurality of turns of coil. Each turn of coil is formed preferably from an impregnated fibrous strip covering each turn of coil. The impregnated fibrous strip forms a cured body of resinous insulation applied to each turn of coil. The cured resinous composition, in one aspect of the present invention, is an epoxy-anhydride resin that has been prereacted with an antioxidant oligomer selected from the group consisting of organophosphorus compounds, sterically-hindered alkylated phenolics, alkyl and aryl thio-esters, alkyl and aryl thio-phosphites, thiazoles, lactones, hydroxylamines, and maleimides. The fibrous strip could comprise glass fibers or micro tape.
In another aspect of the present invention, the organophosphorus compounds comprises one of 2-phenyl-1-1-1,3,2-dioxaphosphepane, Deoxophostone, Vinylphosphonic acid, or Vinylphosphonic acid dimethyl ester. The sterically-hindered alkylated phenolics can be one of 2,6-di-butyl-4-hydroxymethyl phenol, and N-butyl-p-aminophenol. The alkyl and aryl thio-esters could be Trilauryl trithiophosphite. The thiazoles could include 3-(N-salicyloyl)-amino,2,3-thiazole, and the lactone could be one of Benzofuranones or 3-aryl benzo-furan-2-one. The hydroxylamines can be N-(2-hydroxypropyl) ethylenediamine, Hydroxy ethyldiethylenetriamine, or N-(2-hydroxy,2,4,4-trimethylpentyl) diethylene triamine. The maleimides can include 1,1 (Methylene-4, 1-Phenylene) Bis-Maleimide.
In yet another aspect of the present invention, the epoxy-anhydride resin consists essentially of an epoxy-anhydride and styrene. The organo-phosphorus antioxidant oligomer can comprise an oligomer formed with a reaction of one of vinylphosphonic acid or vinylphosphonic acid and its esters with DiglycidylEther of Neo-Pentyl Glycol or DiglycidylEther of 1,4 Butane Diol. The epoxy-anhydride resin, in one aspect, consists essentially of an epoxy resin mixture consisting essentially of a first epoxy resin consisting essentially of a diglycidyl ether of an aliphatic diol and a second epoxy resin selected from the group consisting essentially of bisphenol A epoxy resin, bisphenol F expoxy resins, novolac epoxy resins, glycidyl ester epoxy resins, hydantoin epoxy resins, cycloaliphatic epoxy resins and mixtures thereof, an organic carboxylic acid anhydride, and a chromium (III) acetylacetonate acting as a latent catalyst.